I'm making the move into DSLR photography and would like some advice as to whether I should go for the D80 with the 18-70mm (and probably purchase the 70-300mm later), or the 18-135mm for a more general purpose zoom. I'm sure it depends on the photographer but if I were to go for the first option, would it be realistic to keep swapping the lenses for the extra reach or would it be less of a problem than I think?

This is an option I am considering and I would prefer a single lens. I will probably go for the body and the 18-200mm VR separately but there's just one thing: is there any significant loss in genera quality, given the fact that the lens has such a long focal length?

If you want a single lens to do it all and can afford it, then yes, it's the model to go for.

If you want a twin lens solution, then go for either the 18-55mm plus 55-200mm VR for a budget option, or the 18-70 plus 70-300mm VR for the quality option. If you're going for the D80 though, I'd sway towards either the 18-70 plus 70-300mm VR or the 18-200mm VR if you can afford it.

The 18-135mm is a really nice option as a budget single lens solution, but if you have a higher budget and or don't mind carrying two lenses, there's better choices.